Until now, displays used in the practical applications comprise primarily Cathode Ray Tube Display (CRT), Liquid Crystal (LCD), Vacuum Fluorescent Display (VFD), Plasma Display Panel (PDP), Organic Light Emitting Display (OLED), Field emission display (FED), Electroluminescent Display (ELD), and so on. OLED is a novel flat display, and has advantages including thinness, light weight, wide viewing angle, active light emitting, continuously adjustable emitting colors, low cost, fast response time, low energy consumption, low driving voltage, wide window of operating temperature, simple production process, high luminous efficiency, flexible display, etc., as compared to LCD. Because of unique advantages over other displayers and good application prospect of OLED, it gets substantial attention of industrial and scientific communities.
The current OLED devices comprise sequentially arranged cathode, electron transfer layer, light emitting layer, hole transport layer, anode, and substrate. Of those, the materials of light emitting layer are preferably host-guest doped emitters comprising host light-emitting materials (host materials) and doping light-emitting materials (dopants). Such host-guest doped emitters separate the transport function of carriers from the light-emitting mechanism, and utilize reasonable collocation of energy levels and interfaces between the host and the guest to optimize the properties thereof, thereby optimizing the performances of the devices. Currently, such doped systems have extended from fluorescent systems to phosphorescent systems, and achieve nearly 100% inner quantum efficiency, so as to make the industrialization of OLED possible.
From the viewpoint of energy level requirement, the energy gap of the host materials should be higher than the dopants, namely, the energy is transferred from the host materials to the dopants so that the dopants are excited for luminescence. Moreover, the phosphorescence doped systems further require that the host materials have higher triplet energy level (ET) than the dopants, and otherwise the energy of the phosphorescence will be reversely transferred from the dopants to the host materials. The host materials are generally pure organic compounds, and cannot utilize the energy of triplet excitons. As a result, the luminescent efficiency of OLED will be lost. With respect to blue phosphorescent host materials, it is relatively hard to achieve because the blue phosphorescent host materials themselves have relatively high ET, and the host materials cannot satisfy the requirement until they have higher ET. Thus, it is hard from the viewpoint of molecular design.
It can be seen that the relatively lower triplet energy level of the existing host materials is likely to cause the OLED to have relatively low efficiency.